When preparing OE devices, like OFETs or OPV cells, or organic light-emitting devices, like organic light-emitting diodes (OLED), in particular flexible devices, usually coating or printing techniques are used to apply the layer of the OSC material. Printing formulations that have hitherto been used in prior art for the preparation of OE devices are usually solution based, comprise aromatic or aliphatic organic solvents and tend to have low viscosities. While this approach serves well for spin coating and ink jet printing (IJP) fabrication methods, recently there has been a growing interest in using traditional printing technologies, like flexographic or gravure printing, to fabricate devices. This requires different types of formulations, in particular with respect to the choice of the solvents and optional additives like viscosity enhancers.
Many printing processes use formulations having a medium viscosity (typically from 10 to 1000 cP), including inks for flexographic and gravure printing, but also for hot jetting IJP, electrostatic IJP, soft lithography and variants thereof, or micro-stamping. However, for printing an OSC material efficiently there are several limitations.
Thus, there is a need to get correct fluid properties for printing by a combination of a single solvent or mixed volatile solvents and the functional material alone, or with the minimum addition of additional viscosity modifiers or binders. This typically results in formulations with a viscosity of <10 cP, like for example from tetralin or xylene solutions etc. Flexographic and gravure printing typically require viscosities of >10 cP to get good print quality, and often formulations having a viscosity in the range of 25-100 cP or even up to 5000 cP are used.
Usually preference is given for a solution of the active material. For example, high viscosity polyols which have viscosities in the above-mentioned ranges do not dissolve or are not compatible with the typical OSC materials of interest, because of the hydrophilic nature of these materials. Additionally, they are often incompatible with the substrates and other parts of the fabrication process.
However, the usual viscosity modifiers (for example inorganics like silica, clays etc., or very high Mw polymers, polyalcohols etc.), which raise the viscosity of the useful solvent up to >10 cP, do on the other hand tend to destroy the OE device performance. Additionally, silicas and clays can adversely affect the low shear flow, rendering the inks unsuitable for IJ, gravure and flexographic printing.
A low viscosity (<10 cP) is often required e.g for spin coating and standard IJP techniques, whereas a high viscosity (>1000 cP) is usually required for example for screen printing, offset lithographic printing etc.
U.S. Pat. No. 6,541,300 discloses a process for preparing an OSC film for use in an OE device, by blending an OSC material with a multicomponent solvent blend and applying the resulting mixture to a substrate. It is claimed that the OSC film obtained by using a solvent blend has a higher mobility and the resulting OE device has a higher on/off ratio, compared to an OSC film, or an OE device including the OSC film, which are produced using only one solvent of the multicomponent solvent blend. However, the claimed process is characterized only by its desired result, i.e. a higher mobility and a higher on/off ratio, but not by the means how this result can be achieved. In particular, the document does not provide a clear guidance for the person skilled in the art how to select suitable solvents in order to achieve a workable formulation. The document only mentions that the solvents of the solvent blend should have a combined polarity (φm) from 0.1 to 1.0, wherein the combined polarity is given by the following equation
      ϕ    m    =            ∑              i        =        1            n        ⁢                  ϕ        i            ⁢              χ        i            with n being the number of solvents, φi being the polarity and χi the mole fraction of a single solvent in the solvent blend. Apart from this parameter, however, the document does not provide any limitation or guidance regarding the choice of the OSC compound, and the choice of solvents that are best suitable for a chosen OSC compound. Although the document discloses a list of OSC compounds (including polymers and small molecules) and a list of solvents having a polarity ranging from low to high values, it is not clear if all these solvents will readily dissolve the disclosed OSC compounds. However, it is known that when a solvent does not dissolve an OSC material, the resulting mixture is often not suitable for the preparation of OSC films and OE devices, since the morphology of the resulting OSC film will deteriorate, which will negatively affect the OE device performance. Therefore, in view of the teaching of U.S. Pat. No. 6,541,300, considerable effort is still required to find the appropriate solvents for a given OSC material as disclosed in this document.
WO 03/069959 A1 discloses an OSC film for use in an electroluminescent (EL) device, formed by a wet film process with a composite solution. The composite solution is prepared by dissolving at least two organic compounds in a mixed organic solvent, including at least two organic solvents having different volatility and different solubility for the organic compounds. Again, the document discloses a large list of possible solvents and organic compounds, but does not provide a clear guidance how suitable solvents can be chosen for a given organic compound. Instead, it is merely said that the selection of the at least two organic solvents having different volatility may depend on the property of the organic compounds.
EP 1 416 069 A1 discloses an OSC element comprising a polyacene as OSC material. The document further mentions that the polyacene may be dissolved in a solvent, and that a combination of two or more solvents may be used. However, apart from a list of standard solvents no preference is given to any particular solvent or solvent combination, and no specific guidance is provided regarding the selection of suitable solvents.
WO 2005/041322 A1 discloses an organic polymer solution for use in OE devices comprising at least one organic polymer, a first solvent and a second solvent, wherein the first solvent has a low solubility and faster evaporation rate than the second solvent, and the second solvent has a very low solubility. Also claimed is a method of manufacturing an OE device by depositing the solution on an electrode and allowing it to dry. It is claimed that, due to the different solubility and evaporation rate of the solvents, a substantially uniform polymer layer is formed. However, no specific values or parameter ranges of the solvent properties are given, which could serve as a basis for the selection of suitable solvents.
EP 1 122 793 A2 discloses an organic luminescence device manufactured from an ink comprising an organic EL material and a hydrophobic organic solvent or solvent mixture, wherein the solvents have a dissolving power of at most 5 wt. % of water at room temperature. However, this value does apply to practically all OLED solvents known in prior art and does therefore not consitute a real limitation. It is further mentioned that the ink should have a viscosity of not more than 5000 cP, preferably not more than 100 cP. However, there is no guidance how to select specific solvents to achieve these values. This makes the selection of suitable solvents still difficult, especially when trying to prepare dilute small molecule solutions having high viscosities, without using non-volatile thickening agents.
WO 03/053707 A2 discloses a screen printable EL polymer ink comprising a soluble EL material, an organic solvent having a boiling point between 120 and 200° C., and a viscosity enhancer to maintain a viscosity of above 50 cP. The organic solvent should preferably have a solubility parameter of between 8.8 and 10.0 (cal/cm3)1/2. The majority of viscosity modifiers mentioned in this document are polymers or inorganic particles, as for example disclosed on page 9 or 10. Further reported is the use of “gel retarders” in concentrations from 1 to 20%, which can also be commercially available products, to decrease solvent evaporation and improve ink stability and processability. However, the use of processing additives as suggested in this document is not always desirable in OSC printing inks for the preparation of OE devices, since these processing additives will remain in the OSC layer after removal of the solvent, where they could negatively affect or even destroy the performance of the device. Instead it is more preferable to have only the pure active OSC material left in the OSC layer after drying, without any processing additives. Therefore, apart of the active OSC material the ink should preferably contain only volatile components.
Therefore there is still a need for a process to provide improved formulations and inks of an OSC material suitable for the manufacture of OE devices, especially OFETs and OPV cells, wherein said process allows broad, but precise selection of solvents that have suitable viscosity and do not adversely affect the performance and lifetime of the device. One aim of the present invention is to provide such a process. Another aim is to provide improved OSC formulations obtainable from such a process. Another aim is to provide improved OE devices obtainable from such OSC formulations. Further aims are immediately evident to the person skilled in the art from the following description.
The inventors of the present invention have found these aims can be achieved, and the above-mentioned problems can be solved, by a process of providing OSC formulations as claimed in the present invention. In particular, the inventors of the present invention have found a method to prepare an OSC formulation by selecting a suitable solvent or solvent blend according to its partition ratio (log P) and other parameters like the viscosity and the boiling point. It was found that by appropriate choice of the solvents within specific ranges of these parameters, it is possible to provide improved OSC formulations, wherein the solvents dissolve the OE compounds at useful levels and still have viscosities that are suitable for the desired printing or coating technique. Since the parameters are defined for the pure solvents, i.e. without any non-volatile additives, it is easier for the skilled person to select suitable solvents and prepare OSC coating or printing inks, without the need to use e.g. thickening agents to adjust the viscosity. If desired, coating or printing inks made from these solvents can also be diluted down with lower viscosity solvents to tune the viscosities as needed. Although the use of viscosity enhancing or modifying additives is not necessarily required, they may be added in small amounts so as not to adversely affect the device performance.